The crankshaft, sometimes abbreviated to crank, is the part of an engine or other device that translates reciprocating linear piston motion into rotation. To convert the reciprocating motion into rotation, the crankshaft has “crank throws” or “crankpins”, additional bearing surfaces whose axis is offset from that of the crank, to which the “big ends” of the connecting rods from each cylinder attach.
A crank is an arm attached at right angles to a rotating shaft by which reciprocating motion is imparted to or received from the shaft. It is used to convert circular motion into reciprocating motion, or vice-versa. The arm may be a bent portion of the shaft, or a separate arm or disk attached to it. Attached to the end of the crank by a pivot is a rod, usually called a connecting rod. The end of the rod attached to the crank moves in a circular motion, while the other end is usually constrained to move in a linear sliding motion.
The mechanical advantage of a crank, the ratio between the force on the connecting rod and the torque on the shaft, varies throughout the crank's cycle. Thus a good portion of the force applied to the crank is absorbed at the bearing surfaces and is not turned into a torque. This puts tremendous pressures on the bearing surfaces until such time that the torque angle improves allowing more of the force to be converted into a torque. These losses, most of which are converted into heat, are responsible for the lowered efficiencies of a crankshaft assembly and in particular in combustion engines that use crankshaft principles, such as pistons.